A neutron detector is a key technology which supports neutron utilization techniques.
The neutron detector is used in the security field such as cargo inspection for the discovery of illegal nuclear-related substances; the academic research field such as structural analysis by neutron diffraction; the nondestructive inspection field; the medical field such as boron neutron capture therapy; and the resource exploration field making use of a neutron. Along with the development of neutron utilization techniques in these fields, a higher-performance neutron detector is desired.
In the neutron detector, neutron detection efficiency and discrimination ability between a neutron and a γ-ray (n/γ discrimination ability) are important.
The neutron detection efficiency is the ratio of the number of neutrons counted by a detector to the number of neutrons incident on the detector. When the neutron detection efficiency is low, the absolute number of neutrons to be measured becomes small, thereby reducing measurement accuracy. The reason that the n/γ discrimination ability is important is as follows. γ-rays are existent in the natural world as natural radiation and also produced when a neutron enters the constituent parts of a detector or a substance to be tested. Therefore, when the n/γ discrimination ability is low, a γ-ray is detected as a neutron, thereby impairing the counting accuracy of neutrons.
A known neutron detector detects a neutron based on the following mechanism.
A neutron has high penetrability without any interaction with a substance. Therefore, the neutron is generally detected by making use of a neutron capture reaction. For example, a conventionally known helium 3 detector detects a neutron by making use of a proton and tritium produced by a neutron capture reaction caused by helium 3. This helium 3 detector has high detection efficiency and excellent n/γ discrimination ability. However, since helium 3 is expensive and there is a limit to the amount of its resources, a device substituting this is awaited.
The development of a neutron detector comprising a neutron scintillator as a detector substituting the above helium 3 detector is now under way. The neutron scintillator is a substance which emits fluorescence when a neutron enters the neutron scintillator. The neutron detector can be manufactured by combining this neutron scintillator and a photodetector such as a photomultiplier tube. The performance of the neutron detector comprising a neutron scintillator depends on a substance constituting the neutron scintillator. When a neutron scintillator having a high content of an isotope which is excellent in neutron capture reaction efficiency is used, the neutron detection efficiency becomes high. Examples of the isotope which is excellent in neutron capture reaction efficiency include lithium 6 and boron 10 (U.S. Pat. No. 8,044,367).
In the neutron detector comprising a neutron scintillator, a photodetector which has detected fluorescence emitted from the neutron scintillator outputs a pulse signal so that the number of neutrons is counted based on the intensity (pulse height) of the pulse signal. A predetermined threshold value is set for a pulse height so that when the pulse height is smaller than the threshold value, the output pulse signal is treated as noise whereas when the pulse height is equal to or larger than the threshold value, the pulse signal is counted as a neutron incidence event.
Although the neutron detector comprising a neutron scintillator has an advantage that neutron detection efficiency is high, it has a problem that it is sensitive to a γ-ray and therefore has poor n/γ discrimination ability.
In the field of X-ray detectors, there is proposed a detector which comprises a powder scintillator and a resin and has small dismatch between the refractive index of the above scintillator and the refractive index of the above resin (U.S. Pat. No. 8,338,790 and U.S. Pat. No. 7,608,829). The powder scintillator used in these technologies is a fine powder which is very small in size.